A growing number of senior military and civilian advisors to President Obama support providing defensive weapons to Ukraine as that country comes under increasing attacks by Russian-backed separatist groups.

New discussions about specifically what aid to provide the fledgling Ukrainian forces comes as pro-Russian rebels announced plans to increase their ranks to 100,000 volunteers in coming weeks. Observers say the rebels are unlikely to find so many additional volunteers and this could just be a cover to bring in more Russian troops.

Ceasefire talks broke down over the weekend followed by some of the deadliest fighting to date, with 28 Ukrainian soldiers killed.

Multiple senior U.S. officials confirm that chief among those advocating for a more aggressive support is Gen. Philip Breedlove, NATO’s military commander, as New York Times first reported on Sunday. Defense Secretary Chuck Hagel, who will likely be replaced by the end of the month, also supports sending defensive weapons. Secretary of State John Kerry is expected to discuss the issue with Ukrainian leadership when he visits Kiev on Thursday.

So far the U.S. has provided Ukrainians with mostly non-lethal aide, including night vision goggles, Explosive Ordinance Detection robots, armor and patrol boats. The U.S. has even provided counter-mortar radar systems designed to locate the launch point of incoming mortar rounds.

Sergey Vaganov/AP Photo

A woman resident passes by a burning house in Mariupol, Ukraine, Saturday, Jan. 24, 2015.

But those systems can’t defend against the artillery attacks, which are devastating Ukrainian civilians and military forces. The U.S. has sophisticated defensive systems to locate artillery rounds at its disposal, but providing those systems has proven to be a more delicate military and political dilemma. That’s because most of the artillery fire is coming from inside Russia, while the mortars are coming from the rebels. U.S. officials, discussing the matter on the condition of anonymity, tell ABC News the Obama administration is wary of providing weapons that would directly engage Russian forces, a move that would all but solidify a proxy war with Putin.

"We are constantly assessing our policies in Ukraine to ensure they are responsive, appropriate, and calibrated to achieve our objectives,” the White House said in an emailed statement Monday. “That said, we are particularly concerned about recent escalating separatist violence and separatist attempts to expand the territory they currently occupy further beyond the ceasefire line agreed to in Minsk on September 19, as well as the increasing toll of civilian and military casualties.”

The statement also said the White remains focused on “pursuing a solution through diplomatic means.”

But a new report generated by a group of former senior American officials, including the former NATO commander Adm. James Stavridis, says the U.S. needs to “bolster deterrence in Ukraine” by providing $3 billion in defensive military aid to the Ukrainians over the next three years. The reports says that support should include, among other things, sending unmanned surveillance drones, electronic anti-drone devices, armored Humvees, light armored missiles, and counter-battery radars.